A threshold dose is a dose level below which there is no effect of radiation on the biological response. It is often difficult to distinguish between a threshold and a linear-quadratic dose response where the response changes only slightly at low doses. A threshold model postulates that radiation does not cause the effect at any level below the threshold. Radiation thresholds are generally thought to be limited to acute (short-term) effects that are called deterministic, because they require depletion of certain cells in the body to below a critical number in a given organ or tissue. These effects include radiation sickness (nausea and vomiting), infection and bleeding, and loss of hair.